1. Field of the Invention
The present invention relate to a process for preparing a 2-alkyl-3-aminothiophene derivative useful as an agricultural fungicide or intermediate thereof, and novel 3-aminothiophene derivatives.
2. Description of the Related Art
Japanese Patent Application Laid-Open (JP-A) No. 9-235282 (EP-A 0737682 A1) describes that a certain kind of 2-alkyl-3-aminothiophene derivative has strong control effect against various plant disease damages. For producing this 2-alkyl-3-aminothiophene derivative, there is envisaged a method in which an alkyl group is directly introduced into 2-position of a 3-aminothiophene derivative. For example, Tetrahedron Letters, 34, 5715 to 5718 (1993) teaches that 2-alkyl-3-aminothiophene is obtained by reacting 3-aminothiophene with various aldehydes in the presence of p-toluenesulfonic acid and selenophenol. ##STR2##
(wherein, R' represents an alkyl group.) However, in this literature, there is utterly no description in which 2-alkyl-3-aminothiophene having a secondary alkyl group is synthesized. For obtaining a 2-alkyl-3-aminothiophene derivative having a secondary alkyl group using a synthesis method described in this literature, 3-aminothiophene derivative has to be reacted with ketones. However, when the present inventors substituted an aldehyde for the ketone and conducted this reaction, 3-aminothiophene was decomposed and the intended 2-alkyl-3-aminothiophene derivative or 2-alkenyl-3-aminothiophene derivative could not be obtained (Reference Example 1). Further, this literature includes a problem as an industrial production method since the reaction uses as a reducing agent selenophenol which is not easily obtained industrially.
Further, Tetrahedron 4, 9055 to 9066 (1998) teaches that when an aldehyde having branching at .alpha.-position is reacted with 3-aminothiophene or 3-aminothiophene derivative, an primary alkenyl group is introduced. However, this literature includes utterly no description regarding reactions using a ketone.